This invention relates to ethylene/xcex1-olefin polymer blends. In one aspect, this invention relates to polymer blends comprising two or more ethylene/xcex1-olefin components while in another aspect, this invention relates to blends in which one or more of the components comprises an ethylene/xcex1-olefin/polyene polymer. In yet another aspect, this invention relates to polymer blends of ethylene/xcex1-olefin components in which the ethylene content of one component differs from the ethylene content of at least one other component by at least about 10 weight percent.
Ethylene/xcex1-olefin polymer blends are well known in the art. The blends taught in U.S. Pat. Nos. 4,438,238; 4,722,971; 4,874,820; 4,902,738; 4,937,299; 4,939,217; 5,013,801; 5,236,998; 5,292,845; 5,382,631; 5,494,965; 5,539,076; 5,691,413; 5,728,766; 4,429,079; 4,530,914; 5,605,969; 5,338,589; 5,260,384; 5,478,890; 5,438,100; 5,476,903; 5,703,180; 5,464,905; 5,744,551; 5,747,620 and 5,798,427 are representative, and each of these patents are incorporated herein by reference.
Blends are useful because they provide properties not available from the individual components from which the blend is made. For example, an ethylene/xcex1-olefin polymer with a relatively narrow molecular weight distribution (MWD), e.g., 2 or less, will usually produce a film with good transparency but it will usually process less efficiently than an ethylene/xcex1-olefin polymer alike in all aspects except with a MWD of 3 or more. However, an ethylene/xcex1-olefin polymer with a MWD of 3 or more usually produces a film that is less transparent than a like ethylene/xcex1-olefin polymer with a MWD of 2 or less. Blending the two polymers will usually produce a composition that will produce a film with both desirable transparency and processability. Moreover, depending upon the particular ethylene/xcex1-olefin polymers, the relative proportions of each, the manner in which the polymers are made and/or blended, the properties of interest and a host of other variables, one or more properties of the blend may be more than a simple average of its component parts.
While ethylene/xcex1-olefin polymer blends can be prepared by any one of a number of different processes, generally these processes fall into one of two categories, i.e., post-reactor blending and in-reactor blending. Illustrative of the former are melt extruders into which two or more solid ethylene/xcex1-olefin polymers are fed and physically mixed into a substantially homogeneous composition, and multiple solution, slurry or gas-phase reactors arranged in a parallel array the output from each blended with one another to form a substantially homogeneous composition which is ultimately recovered in solid form. Illustrative of the latter are multiple reactors connected in series, and single reactors charged with two or more catalysts. While each general process category has its own advantages and disadvantages, in-reactor blending is a favored technique for making blends in which component compatibility, i.e., the ability to make a substantially homogeneous blend from the components, is a factor. Generally, forming a substantially homogeneous blend from ethylene/xcex1-olefin polymer components that are less than fully compatible is easier and more successful and cost effective using an in-reactor technique than a post-reactor technique, particularly melt extrusion.
Ethylene/xcex1-olefin polymers and blends of these materials are commercially important because they exhibit and/or impart desirable properties to various products, e.g., films and molded and extruded articles. Properties of frequent interest are low temperature impact strength, compression set, melt strength, shape retention, pellet flow, mechanical strengths and modulus. Depending upon the end use, often one or more of these properties will be more important than the others. Enhancement of these more important properties often requires the use of a blend of ethylene/xcex1-olefin polymers. The industry interest, of course, is in blends in which the properties of primary importance are enhanced without significant diminution of the other properties.
According to this invention, ethylene/xcex1-olefin polymer blends with improved low temperature, pellet flow, compression set, melt strength and/or shape retention properties are prepared by blending a first ethylene/xcex1-olefin polymer component with a second ethylene/xcex1-olefin polymer component, with the proviso that the ethylene content of the first and second ethylene/xcex1-olefin polymer components differ from one another by at least about 10 weight percent. The blends can be made by either post-reactor or in-reactor blending, and the weight ratio of first component to second component can vary widely, typically from between 80:20 to 20:80. One hallmark of this invention is that the enhanced properties of the blend are achieved without significant diminution of other desirable properties of the blend components.